ES2358328T3 - PROCEDURE AND APPLIANCE FOR REDUCING NOx OF EXHAUST GASES INJECTING REDUCING AGENT IN THE AIR OF FIRE. - Google Patents

Abstract

A process for reducing the concentration of nitrogen oxides in a combustion exhaust gas (24) comprising: a. forming a combustion exhaust gas in a combustion zone (12), the combustion exhaust gas comprising nitrogen oxides; b. provide over fire air (28) and droplets of a solution, particles or a gas of a selective reducing agent (34) in a scorching zone (16), the droplets or particles being smaller than 50 microns in size to promote the rapid reduction of nitrogen oxides so that the average life of the droplet in the combustion exhaust gas and the over fire air (28) is equal to the period during which the over fire air (28) is mixed with combustion exhaust gas; C. mixing (34) the over fire air and the selective reducing agent with the combustion exhaust gas in the scorching zone at a temperature above an optimum temperature range for the reduction of nitrogen oxides; d. as the combustion exhaust gas heats the air from over fire and the selective reducing agent to the optimum temperature range (54) from 871 ° C (1600 ° F) to 1093 ° C (2000 ° F) the nitrogen oxides are reduced with the reducing agent, and e. continue to increase the temperature of the over fire air and the selective reducing agent beyond the optimum temperature range with the exhaust gas.

Description

The present invention relates to nitrogen oxide (NOx) emission controls for combustion systems such as boilers, furnaces, incinerators and other large combustion systems (collectively referred to herein as "boilers"). In particular, the invention relates to the reduction of NOx emissions by the selective reduction of nitrogen oxides to molecular nitrogen. 5

Boiler smoke emissions are eliminated or at least greatly reduced through the use of Over Fire Air Technology (OFA). The OFA stages the combustion air so that most of the air flows into a primary combustion chamber of the boiler and a part of the combustion air is diverted to a burning zone connected downstream of the flame. OFA air facilitates the combustion of smoke particles and smoke particle precursors. 10

Other types of air pollutants produced by combustion include nitrogen oxides, mainly NO and NO2. Nitrogen oxides (NOx) are of growing concern due to their toxicity and their function as precursors in acid rain and photochemical smog procedures. There is a felt need for cost-effective techniques to reduce NOx emissions generated by boilers.

A conventional NOx reduction technique is selective non-catalytic reduction (SNCR) that injects a nitrogen agent into the exhaust gas under conditions that produce a non-catalytic reaction to selectively reduce NOx to molecular nitrogen. The NOx reduction is selective because a large part of the molecular oxygen in the exhaust gas is not reduced. In SNCR, a reagent carrying nitrogen, for example, HN3, urea or an amine compound is injected into the exhaust gas stream at an optimum temperature for the reaction of NH2 and NH radicals with NO reducing it to molecular nitrogen. The optimum temperature for such reactions is centered at approximately 1800 ° F 20 (982 ° C). At substantially higher temperatures, the reagent can oxidize to NO. At substantially lower temperatures, the reagent can pass through unreacted exhaust gases, producing unreacted ammonia. An optimal range of temperatures to reduce NOx using SNCR procedures is narrow and generally from about 1600 ° F (871 ° C) to about 2000 ° F (1093 ° C), referring "approximately" to a temperature difference of about 25 degrees. 25

Exhaust gases reach temperatures above 2000 ° F (1093 ° C), but cool when they pass through the boiler again. In order to allow the exhaust gases to cool before the nitrogen agent is released, schemes have been developed for injecting relatively large droplets or particles of the agent into the exhaust gas such as with over-burning air. The large droplets and particles are sized so that they release the nitrogen agent after the exhaust gas cools. See US Pat. No. 6,280,695. Large droplets delay the release of the reagent in the exhaust gas stream until the apparent temperature of the exhaust gas cools to a temperature window of approximately 1600 ° F (871 ° C) to 2000 ° F (1093 ° C).

The present invention is defined in claim 1.

The present invention provides in one embodiment a method for removing nitrogen oxides by injecting reducing agent into a gas stream while simultaneously minimizing the unreacted ammonia. 35

The invention will now be described in greater detail by way of example with reference to the drawings in which:

FIGURE 1 is a schematic side view of a combustion system having a nitrogen agent injector in an over fire air port.

FIGURE 2 is a graph of combustion exhaust gas temperature versus time.

FIGURES 3 to 6 are diagrams showing the effect on NOx emission levels due to the injection of a gaseous nitrogen agent (NH3) at different stoichiometric ratios and different exhaust gas temperatures.

FIGURE 7 is a table of computer generated predictions of NOx emission reductions for various droplet sizes of the nitrogen agent, exhaust gas temperatures and other boiler operating parameters.

FIGURE 8 is a diagram of computer generated predictions of NOx emission reductions for various droplet sizes of nitrogen agent and exhaust gas temperatures. Four. Five

A nitrogen reagent (a “nitrogen agent”) that carries gaseous or small droplets (less than 50 micrometers in diameter) is introduced with stepped combustion air, for example, OFA air, downstream of the primary combustion zone to reduce NOx in the exhaust gas of a boiler. In staggered combustion, a part of the air required to complete the combustion (over-fire air) is injected downstream of the primary combustion process such as where the products of the primary combustion exhaust gas have cooled to approximately 2400 ° F 50 (1316 ° C) at 2600 ° F (1427 ° C). The nitrogen agent in gaseous form or small droplets (approximately <50 micrometers) in an aqueous solution is introduced along with the stepped air into the extremely hot exhaust gas. The stepped combustion air and the nitrogen agent rapidly heat up to the temperature of the exhaust gas at more than 2000 ° F (1093 ° C).

The surprising discovery has been made that the reduction of NOx can be achieved with SNCR by releasing a gaseous nitrogen agent or small droplets in an area at or near that of the OFA injector at which the apparent temperature of the exhaust gas It is too hot for optimal SNCR. A gaseous ammonia nitrogen agent or small droplets, for example, average diameter less than about 50 micrometers, is injected into the OFA system before or simultaneously with mixing OFA air with the NOx-containing exhaust gas. The nitrogen agent gas 5 or the small droplets provide a rapid release of nitrogen reagent such as in a period of less than about 0.1 to 0.3 seconds. Nitrogen release occurs when the mixture of relatively cold nitrogen agent and over-fire air is heated by exhaust gases through a window of optimum temperature for SNCR and at warmer temperatures. The nitrogen may be within the optimum temperature window for a short period, for example, 0.1 to 0.3 seconds. By introducing the nitrogen agent as gas or small droplets, the agent is rapidly released during the short period of the optimum temperature window. The rapid release ensures that the nitrogen agent contacts the NOx in the exhaust gas when the agent is within the optimum temperature window. In addition, the release occurs next to the entrance to the over-fire air injector in which the vigorous mixing between streams of over-fire air and exhaust gas occurs. fifteen

The droplet size of the agent is adjusted so that the average life of the droplet in the exhaust gas and OFA is equal to the period of the optimum temperature window and / or the period during which the over fire air is mixed with combustion exhaust gas. In general, a suitable initial average size of the droplets injected into the over fire air is less than about 50 micrometers in diameter. The preferred droplet size is a droplet size when injected into the over fire air, for example, the droplet size before evaporation. 20 Preferably, the average life of the droplet is less than about one tenth (0.1) to 0.3 of a second.

FIGURE 1 is a schematic representation of a combustion system 10 as used in a boiler heated with coal and adapted for the processes of the present invention. The combustion system 10 includes a combustion zone 12, a post-combustion zone 14 and a scorching zone 16. The combustion zone 12 is equipped with at least one, and preferably a plurality, of main burners 18 which are supplied with a main fuel such as coal through a fuel inlet 20 and with air through an air inlet 22. The main fuel is burned in the combustion zone 12 to form a combustion exhaust gas 24 circulating from the combustion zone towards the scorching zone 16 in a "downstream direction".

If there is no post-combustion zone, then the entire heat source, for example, coal, is injected into the combustion zone by the main burners 18. If a post-combustion zone 14 is included in the combustion system, 30 typically about 70% to 95% of the total heat input is supplied by the main burners 18, and the remaining 5% to 30% of heat is supplied by injecting a post-combustion fuel such as natural gas, coal or oil through an inlet 26 of post-combustion fuel. Downstream of the combustion and post-combustion zones, air 28 of over fire (OFA) is injected through an inlet 30 of air over fire in the area 16 of OFA burning. Downstream from the scorching zone, combustion exhaust gas 24 passes through a series of 35 heat exchangers 32 and a particle control device (not shown) such as an electrostatic precipitator (ESP) or sleeve filters that eliminates solid particles of the exhaust gas such as fly ash.

The combustion exhaust gas 24 is formed by burning conventional fuels, for example coal, in any of a variety of conventional combustion systems. The burn zone 16 is formed by injecting air 28 from over fire into a region of the combustion system connected downstream, that is, in the direction of flow of the combustion exhaust gas from the combustion zone. Combustion devices that include a combustion zone for oxidizing a fuel and a scorching zone can be adapted to receive a mixture of a nitrogen reducing agent and over-fire air as a means to reduce NOx emissions. For example, combustion and burning areas can be provided in a power plant, boiler, furnace, magnetohydrodynamic combustion chamber (MHD), incinerator, engine or other combustion device. Four. Five

A selective reducing agent 34 (nitrogen agent or N agent) is injected into the over fire air before or simultaneously with the injection of the over fire air 28 into the burn zone 16. The nitrogen agent can be injected through a centered nozzle 36 or other injection system at the center of an OFA air flow, for example, connection of the OFA inlet port 30 to the exhaust gas stream. The OFA port 30 may comprise ports of entry in the corners of the boiler and in which the ports are at the same elevation in the boiler. The nozzle 36 50 can inject that nitrogen agent into the OFA inlet to the exhaust gas stream or substantially upstream of the inlet and before the OFA is mixed with the exhaust gas. Aqueous solutions of the selective reducing agent can be injected into OFA air using conventional injection systems commonly used to generate small droplets. Nitrogen agents can be injected by gas-liquid injectors such as various atomizers. Suitable atomizers include dual fluid atomizers that use air or steam as an atomizing medium, in addition to suitably designed pressure atomizers.

The nitrogen agent injection system 36 can provide droplets with an average size that can be adjusted. The initial average size distribution of the spray droplets can be substantially monodispersed, for example, having less than about 10% of the droplet droplet sizes (i.e., diameter) less than about half the average droplet size, and less than about 10% of the droplets having a droplet size greater than about 1.5 times the average droplet size. The average droplet size

injected into the OFA can be determined by selecting droplet sizes that optimize the evaporation time of the nitrogen agent droplets.

The terms nitrogen oxides and NOx are used interchangeably to refer to the chemical species nitric oxide (NO) and nitrogen dioxide (NO2). Other nitrogen oxides such as N2O, N2O3, N2O4 and N2O5 are known, but these species are not emitted in significant amounts by stationary combustion sources (except N2O in some combustion systems). The term nitrogen oxides (NOx) is generally used to encompass all binary N-O compounds; however, NOx also refers particularly to NO and NO2.

The terms selective reducing agent and nitrogen agent are used interchangeably to refer to any of a variety of chemical species that can selectively reduce NOx in the presence of oxygen in a combustion system. In general, suitable selective reducing agents include urea, ammonia, cyanuric acid, hydrazine, tanolamine, biuret, triuret, amelide, ammonium salts of organic acids, ammonium salts of inorganic acids and the like. Specific examples of ammonium salt reducing agents include ammonium sulfate, ammonium bisulfate, ammonium bisulphite, ammonium formate, ammonium carbonate, ammonium bicarbonate and the like. Mixtures of these selective reducing agents may also be used. The selective reducing agent may be provided in a solution, preferably an aqueous solution, or as a gas. A preferred selective reducing agent is urea in aqueous solution. fifteen

Locating a gaseous or small droplet SNCR injection system with the over-fire injection system is generally convenient and avoids reduced and hot space in the primary combustion zone of a boiler or furnace. The use of over-fire air to introduce a nitrogen agent allows the gas and / or droplets of the agent solution to be injected into an exhaust gas containing NOx and then rapidly heated to a temperature appropriate for reduction NOx by SNCR without the costs and downtime of installing an injection system 20 in a high temperature region of the boiler, furnace or other combustion system.

The stoichiometric ratio of the amount of selective reducing agent in the over-fire air with respect to the amount of NOx in the exhaust gas being treated is about 0.4 to about 10, and preferably about 0.7 to about 3. The stoichiometric ratio is the ratio of the number of moles of nitrogen atoms in the selective reducing agent to the number of moles of nitrogen atoms in the NOx. 25

The selective reducing agent may be provided in an aqueous solution in the form of optimized drops or in a gas that is injected into the over fire air before the injection of the over fire air into the post-combustion zone, simultaneously with the injection of the OFA air to the afterburner zone, or both. The nitrogen agent solution or gas can also be injected into the OFA of a combustion system without post-combustion. The aqueous nitrogen agent solution may contain the selective reducing agent in any suitable concentration, such as from about 5% to about 90% by weight. A preferred concentration range for urea is from about 10% to about 50% by weight.

Nitrogen agents can be injected with OFA without previously injecting post-combustion fuel into the exhaust gas. In addition, N agents can be injected with recirculated exhaust gas that can serve the same purpose as OFA. For example, the recirculated exhaust gas enriched with oxygen or air can be injected together with the N 35 agents by the same injector or separate injectors.

FIGURE 2 is a graph 50 showing the residence time of a nitrogen agent in OFA and exhaust gas versus gas temperature. As represented by representation 52 of the average OFA temperature for a nitrogen agent having droplets less than 50 microns, the mixture of OFA nitrogen and air agent increases from about 600 ° F (316 ° C) to 2,500 ° F (1371 ° C ) in approximately 0.4 seconds when the mixture enters the exhaust gas 40 from the OFA port 30. Like the mixture of over-fire air, nitrogen agent and exhaust gas, the nitrogen agent is heated through a temperature window 54 that is optimal for SNCR, for example, 1,600 ° F (871 ° C) at 2,000 ºF (1093 ° C). This temperature window is short, for example, about 0.1 or 0.2 seconds.

During the short window 54 of optimum SNCR temperature, the gaseous or small diameter nitrogen agent 45 reduces substantial amounts of NOx in the exhaust gas. The NOx reduction is further promoted if the OFA and the exhaust gas are mixed vigorously at the same time that the nitrogen agent is heated through the optimum temperature window. Vigorous mixing and rapid heating of OFA generally occur as the over fire air enters the exhaust gas from the OFA inlet port 30. Therefore, the reduction of NOx due to SNCR occurs when the over-fire air enters and mixes with the exhaust gas. In addition, it appears that the injection of the nitrogen agent through a nozzle 36 at or near the OFA inlet port and / or aligned with the center of the over fire air stream promotes the reduction of NOx in the combustible gas . The OFA air that protects the nitrogen agent initially reacts with any residual carbon monoxide (CO) that would otherwise interfere with the chemistry of the SNCR.

The nitrogen agent and OFA are rapidly heated by the exhaust gas (see representation 55) at a temperature outside the optimal SNCR window 54. Much of the NOx in the exhaust gas has already been reduced when the OFA and any remaining nitrogen agent are heated to the temperature of the exhaust gas of 2,500 ° F (1371 ° C) (see where representation 52 of the OFA converges with the representation 55 of the exhaust gas) which is too hot for the

Optimal SNCR. As the combustion gas 55 cools to 2,000 ° F (1093 ° C) and below, it flows past the nose plane 56 of the combustion system, for example, a boiler, and into the supersaturated steam unit (SSH) 58 and the steam reheating unit 60 (RH) that receives the exhaust gas leaving the boiler.

The NOX performance tests with injection of gaseous ammonia in over-fire air were performed in a boiler simulator installation (BSF) of 1.0 MMBTU / h that provides an accurate subscale simulation of the 5 temperatures and gas compositions Exhaust found in a full-scale boiler. The BSF is described in US Pat. No. 6,280,695. For testing, the BSF was ignited with natural gas. A specially constructed over fire air injector was placed at a specific exhaust gas temperature. The injector consisted of an axial nitrogen carrier tube with an ID of 1,875 inches (4.76 cm) surrounded by an annular fire air tube with an internal diameter (ID) of 0.25 inches (0.635 cm). Ammonia was added to both the nitrogen carrier current and the over-fire air to reduce NOx emissions. The injector was aligned with the central line of the BSF and pointed down (that is, in parallel with the exhaust gas).

The stoichiometric ratio of the primary flame (RE1) in the tests was 1.0 and 1.05. Sufficient OFA was injected to maintain a constant final RE (RE3) at 1.20 which corresponds to approximately 3% O2 in excess in the exhaust gas. These conditions were defined as the initial level. The BSF burner system in the initial conditions 15 generated controlled initial NOx levels of 185 ppm and 205 ppm at 0% O2 to RE1 equal to 1.0 and 1.05, respectively. RE1 is the stoichiometric ratio of air to fuel in zone 12 of primary combustion; RE2 is the same ratio, but in the post-combustion zone 14, and RE3 is the stoichiometric ratio in the scorching zone 16. The OFA was injected at the exhaust gas temperatures of 2450 ° F (1343 ° C) and 2350 ° F (1288 ° C). The gaseous ammonia was injected at a concentration ratio (REN) of 1.5. The concentration ratio (REN) is the ratio of moles of 20 nitrogen atoms in the ammonia with respect to moles of nitrogen atoms in NOx.

The NOx emission levels for Initial Level, NH3 Carrier and OF3 NH3 shown in the diagrams in Figures 3 to 6 refer to tests performed in the BSF under similar conditions, except that the Initial Level bar refers to tests performed without nitrogen agent; The NH3 Carrier bar refers to the injection of the nitrogen agent gas (NH3) together with the nitrogen as a carrier into a central pipe injector that discharges the agent in the center of the over fire air when both enter the Exhaust gas, and OFA NH3 bar refers to the injection of a nitrogen agent gas with the air over fire before they are mixed together with the exhaust gas.

FIGURES 3 to 6 show the effect of the injection of gaseous ammonia by the OFA port on NOx emissions. The initial NOx concentration was smaller for RE1 equal to 1.0 than when RE1 was equal to 1.05 as shown by a bar chart comparison of the test data taken at RE1 = 1.0 shown in the Figure 3 with the diagram data taken at RE1 = 1.05 shown in Figure 4. A similar comparison is made with respect to Figures 5 and 6. The test results show that the injection of gaseous ammonia (NH3) into OFA produced from 20% to 45% reduction of NOx, which depends on the temperature of the exhaust gas at the point of injection of over-fire air and the stoichiometry of the main burner (RE1). Better performance was achieved when the over fire air was injected at cooler exhaust gas temperatures. Injection of NH3 through the center of the OFA 35 port using nitrogen gas as a carrier provided a slightly better reduction of NOx than the injection of ammonia (NH3) into the over fire air stream before the OFA stream mixed with the exhaust gas.

A computational fluid dynamics (CFD) analysis was performed to predict SNCR performance. The CFD model solved the transport equations for continuity, timing, energy and species. Appropriate models were applied to solve the turbulence, radiation, discrete phase trajectories and combustion. For the CFD study, a tangential boiler of 160 MW was evaluated with a nitrogen reagent injected into the air injectors over fire at different operating conditions. Before simulating the nitrogen reagent injection procedure, the CFD model was validated using initial field test data and average temperature profiles of a proprietary thermal model for full load operating conditions. The flow profiles of the exhaust gas in the injector region of the model were based on those of the physical flow modeling test results. Four. Five

The CFD model was run for various operating conditions to investigate the impact of the reagent droplet size, the ignition speed reflected by the gas temperature immediately below the OFA injector, the CO concentration below the OFA injector and the stoichiometric ratio of the nitrogen reagent with respect to the initial NOx nitrogen (REN) in the SNCR NOx trimming yield. In this case, NOx trimming refers to the reduction of NOx that exceeds that of air conditions over pure fire. fifty

Figures 7 and 8 show the predicted NOx trimming performance (emission reduction) from the CFD model at different boiler process conditions. The NOx reduction procedure is more effective for exhaust gas temperatures below about 2500 ° F (1371 ° C) at the OFA injection port and at a concentration of zero in CO. For the exhaust gas temperatures (at the OFA port) below 2500 ºF (1371 ºC) and CO concentrations of 200 ppmv and below, the NOx cut increases (resulting in a reduction of the NOx emission 55) tailored which decreases the size of the nitrogen agent droplets, which indicates that the nitrogen reagent released near the OFA injectors in the exhaust gas / OFA mixing zone (eg, burn zone 16) is effective in the NOx reduction

Both pilot scale experiments and CFD results show that the injection of small droplets (which

have an average diameter of less than about 50 to 60 micrometers) in the over-fire air allows the release of reagent in the over-fire / exhaust gas mixture to improve NOX clipping with respect to droplet sizes greater than about 50 micrometers

Claims (7)

1. A process for reducing the concentration of nitrogen oxides in a combustion exhaust gas (24) comprising: to. forming a combustion exhaust gas in a combustion zone (12), the combustion exhaust gas comprising nitrogen oxides; 5 b. provide over fire air (28) and droplets of a solution, particles or a gas of a selective reducing agent (34) in a scorching zone (16), the droplets or particles being smaller than 50 microns in size to promote the rapid reduction of nitrogen oxides so that the average life of the droplet in the combustion exhaust gas and the over fire air (28) is equal to the period during which the over fire air (28) is mixed with combustion exhaust gas; 10 C. mixing (34) the over fire air and the selective reducing agent with the combustion exhaust gas in the scorching zone at a temperature above an optimum temperature range for the reduction of nitrogen oxides; d. as the combustion exhaust gas heats the air from over fire and the selective reducing agent to the optimum temperature range (54) from 871 ° C (1600 ° F) to 1093 ° C (2000 ° F) the nitrogen oxides are reduced with the reducing agent, and and. continue to increase the temperature of the over fire air and the selective reducing agent beyond the optimum temperature range with the exhaust gas. 2. The method of claim 1, wherein the optimum temperature range (54) during step (d) occurs in a short period of less than 0.3 seconds and the reduction of nitrogen oxides occurs during The short period. twenty 3. The method of claim 1, wherein the small average size of droplets or particles (34) is not greater than 50 micrometers. 4. The method of claim 1, wherein the step of mixing the over fire air and the selective reducing agent with the exhaust gas occurs when the combustion exhaust gas is in a temperature range of 1371 ° C. (2500 ° F) at 1093 ° C (2000 ° F). 25 5. The method of claim 1, wherein the step of providing the over fire air (28) and the selective reducing agent (34) comprises adding the selective reducing agent to the over fire air simultaneously with the air injection from over fire to combustion exhaust gas in the burning zone. 6. The method of claim 1, wherein the step of providing the over fire air (28) and the selective reducing agent (34) comprises adding the selective reducing agent to the over fire air before injecting the air from over fire 30 in the burning zone. 7. The method of claim 1, wherein the selective reducing agent (34) reduces nitrogen oxides when the exhaust gas is at an average temperature greater than 1093 ° C (2000 ° F) and the exhaust gas mixture , over fire air and reducing agent is at an average temperature of 871 ° C (1600 ° F) to 1093 ° C (2000 ° F).